Hello messages are used as a form of greeting to discover other routers on its networks and establish relationships between neighboring devices. On broadcast or NBMA networks, Hello messages can also be used to elect a DR or a BDR.
DD packets describe the summary LSDB information by carrying LSA headers.
The intervals at which two routers send Hello messages must be consistent; otherwise, cannot be established relationships between neighboring devices.
DD packets contain headers of all LSAs for periodic LSDB synchronization between OSPF neighbors.
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